Failsafe hydraulic control system for vehicle automatic transmission

ABSTRACT

A hydraulic control system for a vehicle automatic transmission, in which hydraulic pressure of a pressure source is supplied through a corresponding one of hydraulic lines to each of engaging means when achieving different gear ratios, and a plurality of shift devices are coupled respectively to the lines for switching supply and discharge of the hydraulic pressures to and from the respective engaging means in accordance with running conditions of the vehicle. At least one valve device is coupled to at least one of the lines. The valve device is movable between a supply position where hydraulic pressure is suppled to a corresponding one of the engaging means and a discharge position where the hydraulic pressure suppled to the engaging means is discharged. A plurality of pressure detectors respectively detect hydraulic pressures in the respective hydraulic lines. When the number of engaging means having applied respectively thereto hydraulic pressures higher than their respective predetermined values is equal to or greater than a predetermined value, a switching device switches the valve devices to move same to its discharge position, thereby avoiding locking of the transmission mechanism. A pressure regulating valve device is interposed between the pressure source and a high speed engaging means which can achieve shifting into a high gear ratio.

BACKGROUND OF THE INVENTION

The present invention relates to hydraulic control systems for vehicleautomatic transmissions and, more particularly, to a hydraulic controlsystem for a transmission in which a plurality of friction engagingmeans are selectively engaged by hydraulic pressure, thereby achievingdesired gear ratios.

An automatic transmission for a vehicle is known from U.S. Pat. No.3,754,482, which comprises first, second, third and fourth engagingmeans. A first shift device is provided in a hydraulic linecommunicating a source of hydraulic pressure with the first engagingmeans, for switching supply and discharge of the hydraulic pressure toand from the first engaging means in accordance with running conditionsof the vehicle. A second shift device is provided in a hydraulic linecommunicating the hydraulic pressure source with the second engagingmeans, for switching supply and discharge of the hydraulic pressure toand from the second engaging means in accordance with the runningconditions of the vehicle. A third shift device is provided in ahydraulic line communicating the hydraulic pressure source with thethird engaging means, for switching supply and discharge of thehydraulic pressure to and from the third engaging means in accordancewith the running conditions of the vehicle. A fourth shift device isprovided in a hydraulic line communicating the hydraulic pressure sourcewith the fourth engaging means, for switching supply and discharge ofthe hydraulic pressure to and from the fourth engaging means inaccordance with the running conditions of the vehicle. Two of the firstthrough fourth engaging means are selectively brought to theirrespective engaged positions, whereby a plurality of gear ratios can beachieved. The arrangement of the automatic transmission disclosed in theabove-mentioned U.S. patent is such that supply and discharge of thehydraulic pressure to and from each of the engaging means are switchedby ON and OFF operations of a corresponding one of solenoid valvesconnected respectively to spools of the respective shift devices.Malfunction of the solenoid valves or malfunction of an electroniccontrol system for controlling the solenoid valves would bring one ortwo engaging means other than a predetermined combination of engagingmeans to the engaged position or positions simultaneously with movementof the predetermined combination of engaging means to their respectiveengaged positions. This would cause an inconsistency to occur in atorque transmission path of a gear train in the automatic transmission,so that input and output shafts are locked from rotation or thetransmission is damaged. In order to avoid such deficiency, a relayvalve is provided in each of the hydraulic lines communicating thehydraulic pressure source with the respective shift devices.

For example, when the solenoid valve is operated, which drives the shiftdevice supplying the hydraulic pressure or line pressure to the 1st gearratio/reverse engaging means, the line pressure is introduced into therelay valve provided in the hydraulic line between the second shiftdevice and the hydraulic pressure source, to forcibly move the spool ofthe relay valve to a switching position, thereby intercepting thehydraulic line connecting the hydraulic pressure source to the 2nd gearratio shift device. This prevents, the line pressure from being suppliedto the 2nd gear ratio engaging means through the 2nd gear ratio shiftdevice, even if the solenoid valve for driving the 2nd gear ratio shiftdevice is operated due to malfunction or the like of the electroniccontrol system.

Thus, it is possible for the automatic transmission disclosed in theabove-mentioned U.S. Pat. No. 3,754,482 to avoid such an inconvenientsituation that the hydraulic pressure is simultaneously supplied to theengaging means for achieving their respective gear ratios different fromeach other so that the gears of the transmission are damaged or lockedfrom rotation.

It is required for the arrangement disclosed in the aforesaid U.S.patent, however, to provide the relay valve in each of the hydrauliclines communicating the hydraulic pressure source with the respectiveshift devices, in order to achieve the above-mentioned object. By thisreason, the hydraulic circuit becomes complicated in structure, and thenumber of component parts increases, resulting in a rise in themanufacturing cost and in a rise in the probability of occurrence ofdefects such as valve sticking and the like.

Apart from the above, a technique is known from Japanese PatentPublication No. 48-209 corresponding to U.S. Pat. No. 3,684,066, inwhich engaging forces acting upon respective engaging means are variedcorrespondingly to gear ratios. That is, in the low gear ratio in whichtransmission torque is relatively high, the engaging forces acting onthe respective engaging means are raised to increase the torquecapacity, while in the high gear ratio in which the transmission torqueis relatively low, the engaging forces acting upon the respectiveengaging means are reduced to bring the torque capacity to a levelcorresponding to the transmission torque, thereby reducing drivinglosses of a hydraulic pump.

The arrangement disclosed in the above-mentioned Japanese patent is suchthat, at shifting of gear ratios, switching is made from relatively highhydraulic pressure supplied to a low speed engaging means to relativelylow hydraulic pressure supplied to a high speed engaging means, therebyvarying the torque capacity of the engaging means. The arrangement isinconvenient, however, in that shifting shocks occur, because theswitching of the hydraulic pressure is made simultaneously withswitching of the shift devices, that is, at the early stage of the gearratio shifting. In order to dissolve the inconvenience, control of thehydraulic pressure supplied to the engaging means for achieving shiftingtoward the high gear ratio during the gear ratio shifting cannot butbecome complicated.

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a hydraulic control systemfor a vehicle automatic transmission, which can dissolve theabove-discussed inconveniences, and which is simple in construction,high in safety, and low in shifting shocks, so that the vehicle iscomfortable to ride in.

It is a more specific object of the invention to provide a hydrauliccontrol system for a vehicle automatic transmission, which is providedwith a safety mechanism which, with a requisite minimum increase in thenumber of component parts, can prevent such an inconvenient situationthat one or more engaging means other than engaging means to be engagedin order to achieve a required gear ratio are engaged due to malfunctionor the like of a control system, to lock a transmission mechanism sothat wheels of the vehicle during running are suddenly locked fromrotation or the vehicle is made entirely impossible to run.

It is another object of the invention to provide a hydraulic controlsystem for a vehicle automatic transmission, which, with a requisiteminimum increase in the number of component parts, can restrain a risein the manufacturing cost and a rise in the probability of occurrence ofdefects such as valve sticking and the like in a hydraulic circuit,thereby enabling the hydraulic circuit arrangement to be simplified inconstruction.

It is still another object of the invention to provide a hydrauliccontrol system for a vehicle automatic transmission, which can bringengaging forces acting upon respective engaging means achievingrequisite shifting into their respective gear ratios to respectivelevels corresponding to the respective gear ratios, to restrain themagnitude of the hydraulic pressure supplied to the engaging means tothe requisite minimum, thereby preventing occurrence of noises andreducing losses of the engine output for driving a hydraulic pump, andwhich is easy in control of the engaging forces during shifting of gearratios so that an attempt is made to reduce shifting shocks.

According to the invention, there is provided a hydraulic control systemfor a vehicle automatic transmission which comprises first and secondengaging means respectively engagable at different gear ratios,hydraulic pressure supplying means for supplying hydraulic pressure tocause engagement of each of the first and second engaging means, firstand second hydraulic passage means for connecting the hydraulic pressuresupplying means respectively to the first and second engaging means, andfirst and second shift means provided respectively in the first andsecond hydraulic passage means for switching supply and discharge of thehydraulic pressure to and from the respective first and second engagingmeans in accordance with running conditions of the vehicle.

First and second pressure detecting means of the hydraulic controlsystem respectively detect hydraulic pressure in the first hydraulicpassage means between the first shift means and the first engaging meansand hydraulic pressure in the second hydraulic passage means between thesecond shift means and the second engaging means. Valve means is coupledto the first hydraulic passage means between the first pressuredetecting means and the first engaging means and is movable between asupply position where the valve means opens the first hydraulic passagemeans for supplying the hydraulic pressure to be supplied to the firstengaging means and a discharge position where the valve means dischargesthe hydraulic pressure supplied to the first engaging means. Switchingmeans is coupled to the valve means for switching the valve means tomove the same to the discharge position when the first pressuredetecting means detects that the hydraulic pressure in the firsthydraulic passage means is higher than a first predetermined value andthe second pressure detecting means detects that the hydraulic pressurein the second hydraulic passage means is higher than a secondpredetermined value. With such arrangement, it is possible to avoid suchan inconvenient situation that a transmission mechanism falls into alocked condition.

Preferably, the valve means comprises a spool valve which includes aspool and a plurality of lands integrally formed on the spool. The landshave their respective pressure receiving sections which include a firstpressure receiving section having applied thereto the hydraulic pressuresupplied from the first shift means to the first engaging means, forbiasing the spool toward the discharge position, a second pressurereceiving section having applied thereto the hydraulic pressure suppliedfrom the second shift means to the second engaging means, for biasingthe spool toward the discharge position, and a third pressure receivingsection having normally applied thereto hydraulic pressure of apredetermined magnitude, for biasing the spool toward the supplyposition. The spool is switched to the discharge position when thehydraulic pressure acting upon the first pressure receiving sectionexceeds the first predetermined value and the hydraulic pressure actingupon the second pressure receiving section exceeds the secondpredetermined value.

According to another aspect of the invention, there is provided ahydraulic control system for a vehicle automatic transmission whichcomprises first, second and third engaging means, the engaging meansbeing combined to form a plurality of sets of pairs, the engaging meansin each set being simultaneously engaged to enable shifting into aplurality of gear ratios corresponding in number to the sets to beachieved, hydraulic pressure supplying means for supplying hydraulicpressure to cause engagement of each of the first, second and thirdengaging means respectively, first, second and third hydraulic passagemeans for connecting the hydraulic pressure supplying means respectivelyto the first, second and third engaging means, and first, second andthird shift means coupled respectively to the first, second and thirdhydraulic passage means for switching supply and discharge of thehydraulic pressure to and from the respective first, second and thirdengaging means in accordance with running conditions of the vehicle.

First, second and third pressure detecting means of the hydrauliccontrol system respectively detect hydraulic pressure in the firsthydraulic passage means between the first shift means and the firstengaging means, hydraulic pressure in the second hydraulic passage meansbetween the second shift means and the second engaging means, andhydraulic pressure in the third hydraulic passage means between thethird shift means and the third engaging means. Valve means is coupledto the first hydraulic passage means between the first pressuredetecting means and the first engaging means and is movable between asupply position where the valve means opens the first hydraulic passagemeans for supplying the hydraulic pressure to be supplied to the firstengaging means and a discharge position where the valve means dischargesthe hydraulic pressure supplied to the first engaging means. Switchingmeans is coupled to the valve means for switching the valve means tomove the same to the discharge position when the first, second and thirdpressure detecting means simultaneously detect that the hydraulicpressures in the respective first, second and third hydraulic passagemeans are respectively higher than first, second and third predeterminedvalues.

According to still another aspect of the invention, there is provided ahydraulic control system for a vehicle automatic transmission whichcomprises first, second and third engaging means respectively engagablewhen achieving three different forward gear ratios, fourth engagingmeans engagable when achieving each of three forward gear ratios,hydraulic pressure supplying means for supplying hydraulic pressure tocause engagement of each of the first, second, third and fourth engagingmeans respectively, first through fourth hydraulic passage means forconnecting the hydraulic pressure supplying means respectively to thefirst through fourth engaging means, and first through third shift meanscoupled respectively to the first through third hydraulic passage meansfor switching supply and discharge of the hydraulic pressure to and fromthe respective first through third engaging means in accordance withrunning conditions of the vehicle.

First through fourth pressure detecting means of the hydraulic controlsystem respectively detect hydraulic pressure in the first hydraulicpassage means between the first shift means and the first engagingmeans, hydraulic pressure in the second hydraulic passage means betweenthe second shift means and the second engaging means, hydraulic pressurein the third hydraulic passage means between the third shift means andthe third engaging means, and hydraulic pressure in the fourth hydraulicpassage means. First valve means is provided in a portion of any one ofthe first, second and third hydraulic passage means, which portionextends between the pressure detecting means coupled to the hydraulicpassage means and the engaging means connected to the hydraulic passagemeans. The first valve means is movable between a supply position wherethe first valve means opens the hydraulic passage means for supplyingthe hydraulic pressure to be supplied to the engaging means and adischarge position where the first valve means discharges the hydraulicpressure supplied to the engaging means. Second valve means is providedin a portion of any one of the second, third and fourth hydraulicpassage means in which hydraulic passage means the first valve means isnot provided. The portion of any one of the second, third and fourthhydraulic passage means extends between the pressure detecting meanscoupled to the hydraulic passage means and the engaging means connectedto the hydraulic passage means. The second valve means is movablebetween a supply position where the second valve means opens thehydraulic passage means for supplying the hydraulic pressure to besupplied to the engaging means and a discharge position where the secondvalve means discharges the hydraulic pressure supplied to the engagingmeans. First switching means is coupled to the first valve means forswitching the first valve means to move the same to the dischargeposition when the pressure detecting means coupled to the hydraulicpassage means having provided therein the first valve means among thefirst, second and third hydraulic passage means detects that thehydraulic pressure in the hydraulic passage means is higher than apredetermined value and the pressure detecting means provided in atleast one of the remaining hydraulic passage means detects that thehydraulic pressure in the hydraulic passage means is higher than apredetermined value. Second switching means is coupled to the secondvalve means for switching the second valve means to move the same to thedischarge position when the pressure detecting means providedrespectively in the second, third and fourth hydraulic passage meanssimultaneously detect that the hydraulic pressures in the respectivehydraulic passage means are higher than their respective predeterminedvalues.

According to another aspect of the invention, there is provided ahydraulic control system for a vehicle automatic transmission whichcomprises first through fourth engaging means, hydraulic pressuresupplying means for supplying hydraulic pressure to cause engagement ofeach of the first through fourth engaging means respectively, firstthrough fourth hydraulic passage means for connecting the hydraulicpressure supplying means respectively to the first through fourthengaging means, and first through fourth shift means coupledrespectively to the first through fourth hydraulic passage means forswitching supply and discharge of the hydraulic pressure to and from therespective first through fourth engaging means in accordance withrunning conditions of the vehicle, the arrangement being such that a 1stgear ratio is achieved when the first and fourth engaging means aresimultaneously engaged, a 2nd gear ratio is achieved when the second andfourth engaging means are simultaneously engaged, a 3rd gear ratio, forexample, a directly coupled gear ratio in which an input shaft and anoutput shaft of the transmission are substantially equal in rotationalspeed to each other is achieved when the third and fourth engaging meansare simultaneously engaged, a 4th gear ratio, for example, an overdrivegear ratio in which the output shaft is higher in rotational speed thanthe input shaft is achieved when the second and third engaging means aresimultaneously engaged.

First through fourth pressure detecting means of the hydraulic controlsystem respectively detect hydraulic pressure in the first hydraulicpassage means between the first shift means and the first engagingmeans, hydraulic pressure in the second hydraulic passage means betweenthe second shift means and the second engaging means, hydraulic pressurein the third hydraulic passage means between the third shift means andthe third engaging means, and hydraulic pressure in the fourth hydraulicpassage means between the fourth shift means and the fourth engagingmeans. First valve means is provided in a portion of any one of thefirst, second and third hydraulic passage means, which portion extendsbetween the pressure detecting means coupled to the hydraulic passagemeans and the engaging means connected to the hydraulic passage means.The first valve means is movable between a supply position where thefirst valve means opens the hydraulic passage means for supplying thehydraulic pressure to be supplied to the engaging means and a dischargeposition where the first valve means discharges the hydraulic pressuresupplied to the engaging means. Second valve means is provided in aportion of any one of the second, third and fourth hydraulic passagemeans in which hydraulic passage means the first valve means is notprovided. The portion of any one of the second, third and fourthhydraulic passage means extends between the pressure detecting meanscoupled to the hydraulic passage means and the engaging means connectedto the hydraulic passage means. The second valve means is movablebetween a supply position where the second valve means opens thehydraulic passage means for supplying the hydraulic pressure to besupplied to the engaging means and a discharge position where the secondvalve means discharges the hydraulic pressure supplied to the engagingmeans. First switching means is coupled to the first valve means forswitching the first valve means to move the same to the dischargeposition when the pressure detecting means coupled to the hydraulicpassage means having provided therein the first valve means detects thatthe hydraulic pressure in the hydraulic passage means is higher than apredetermined value and the pressure detecting means coupled to at leastone of the remaining hydraulic passage means detects that the hydraulicpressure in the hydraulic passage means is higher than a predeterminedvalue. Second switching means is coupled to the second valve means forswitching the second valve means to move the same to the dischargeposition when the pressure detecting means coupled respectively to thesecond, third and fourth hydraulic passage means simultaneously detectthat the hydraulic pressures in the respective hydraulic passage meansare higher than their respective predetermined values.

According to still another aspect of the invention, there is provided ahydraulic control system for a vehicle automatic transmission whichcomprises a plurality of engaging means including low speed engagingmeans for achieving a relatively low gear ratio and high speed engagingmeans for achieving a relatively high gear ratio, and first hydraulicpressure supplying means for supplying the hydraulic pressure to causeengagement of each of the engaging means.

Second hydraulic pressure supplying means of the hydraulic controlsystem generates a signal hydraulic pressure. Pressure regulating valvemeans is arranged between the high speed engaging means and the firsthydraulic pressure supplying means for reducing the hydraulic pressuresupplied to the high speed engaging means when the signal hydraulicpressure is supplied from the second hydraulic pressure supplying means.Switching valve means is provided which is movable between a supplyposition where the switching valve means enables the signal hydraulicpressure of the second hydraulic pressure supplying means to be suppliedto the pressure regulating valve means and a discharge position wherethe switching valve means discharges the signal hydraulic pressuresupplied to the pressure regulating valve means. The arrangement is suchthat the switching valve means is switched to the supply position whenthe hydraulic pressure supplied from the first hydraulic pressuresupplying means to the high speed engaging means is higher than apredetermined value. With such arrangement, losses of the engine outputfor causing the hydraulic pressure supplying means to generate thesupply hydraulic pressure are reduced, whereby an attempt can be made toreduce shifting shocks.

The above and other objects, features and advantages of the inventionwill become more apparent from the ensuing detailed description taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view of a power train of a vehicle automatictransmission to which a hydraulic control system according to theinvention is applied; and

FIGS. 2, 2A, 2B and 2C are hydraulic circuit diagrams of the hydrauliccontrol system for the vehicle automatic transmission according to theinvention, FIG. 2 showing the arrangement of FIGS. 2A-2C relative toeach other.

DETAILED DESCRIPTION

FIG. 1 is a skeletal view showing a speed change gear of an automatictransmission which can achieve shifting into gear ratios including fourforward gear ratios and one reverse gear ratio. The speed change gearcomprises a drive shaft 10 which is adapted to be connected directly toa crankshaft of an engine (not shown). The drive shaft 10 is connectedto a pump 16 of a torque converter 12 through an input casing 14thereof. The torque converter 12 has a stator 18 which is connected to atransmission casing 22 through a one-way clutch 20. The torque converter12 also has a turbine 24 which is connected, through an input shaft 26,to an overdrive clutch 28 serving as a third engaging means, anunderdrive clutch 30 serving as a fourth engaging means, and a reverseclutch 32. The overdrive clutch 28 has an output side which is connectedto a first carrier 38 of first simple planetary gears 36 (hereinafter,referred to simply as "first gear unit 36"). The first carrier 38 isconnected to a second carrier 42 of second simple planetary gears 40(hereinafter, referred to simply as "second gear unit 40") through afirst intermediate shaft 34, and is rotatable together with the secondcarrier 42. The first carrier 38 is also connected, through theintermediate shaft 34, to a low/reverse brake 44 serving as a firstengaging means for stopping rotation of the intermediate shaft 34. Theunderdrive clutch 30 has an output side which is connected to a firstsun gear 46 of the first gear unit 36. The reverse clutch 32 has anoutput side which is connected to a first ring gear 50 of the first gearunit 36 and to a second sun gear 52 of the second gear unit 40 through asecond intermediate shaft 48. The output side of the reverse clutch 32is also connected to a 2-4 brake 54 serving as a second engaging meansfor stopping rotation of the second intermediate shaft 48.

The first gear unit 36 is composed of the above-mentioned first sun gear46, a first pinion gear 56 in mesh with the sun gear 46, the firstcarrier 38 which rotatably supports the pinion gear 56 and which isrotatable per se, and the above-mentioned first ring gear 50 in meshwith the first pinion gear 56. The second gear unit 40 is composed ofthe above-mentioned second sun gear 52, a second pinion gear 58 in meshwith the sun gear 52, the above-mentioned second carrier 42 whichrotatably supports the pinion gear 58 and which is rotatable per se, anda second ring gear 60 in mesh with the second pinion gear 58. The secondring gear 60 is connected to an output gear 64 through a hollow outputshaft 62 through which the first intermediate shaft 34 extends.

The output gear 64 is in mesh with a driven gear 68 through an idler 70.The driven gear 68 is mounted on a right-hand end of an intermediatetransmission shaft 66 which is arranged substantially in parallelrelation to the input shaft 26. The intermediate transmission shaft 66has a left-hand end thereof which is connected to a final reduction gear76 which is in turn connected to a drive axle 74 through a differentialgear unit 72.

As will be clear from FIG. 1, the transmission casing 22 is so formed asto enclose various power transmission means from the torque converter 12to the output shaft 64, as well as the intermediate transmission shaft66, the differential gear unit 72 and the like.

The above-described clutches and brakes are provided with theirrespective engaging piston devices, servo devices or the like, and aremovable between respective engaged positions and respective released ordisengaged positions, in response to supply and discharge of hydraulicpressure. The hydraulic pressure is supplied by a hydraulic controlsystem illustrated in FIG. 2, selectively to the clutches and brakes.Operative combinations of the clutches and brakes enable shifting intofour forward gear ratios and one reverse gear ratio to be achieved.

The below table 1 indicates the relationship between operations of theclutches and brakes and shifting of gear ratios. In the table 1, thesymbol "o" represents engagement of the clutches and brakes, while thesymbol "-" represents release thereof.

                  TABLE 1                                                         ______________________________________                                        Gear Ratio      1st   2nd    3rd  4th Reverse                                 ______________________________________                                        Reverse Cluth 32                                                                              --    --     --   --  o                                       Low/Reverse Brake 44                                                                          o     --     --   --  o                                       2-4 Brake 54    --    o      --   o   --                                      Underdrive Clutch 30                                                                          o     o      o    --  --                                      Overdrive Clutch 28                                                                           --    --     o    o   --                                      ______________________________________                                    

In the arrangement described above, as the low/reverse brake 44 and theunderdrive clutch 30 are brought to their respective engaged positions,the first and second carriers 38 and 42 are fixed and serve respectivelyas reaction elements. Thus, the driving force from the drive shaft 10 istransmitted to the output shaft 62 through the torque converter 12, theinput shaft 26, the underdrive clutch 30, the first sun gear 46, thefirst pinion gear 56, the first ring gear 50, the second sun gear 52,the second pinion gear 58 and the second ring gear 60. The drive forceis further transmitted to the drive axle 74 through the output gear 64,the intermediate transmission shaft 66 and the final reduction gear 76,so that shifting into the 1st gear ratio is achieved, as will also beapparent from the table 1.

As the brake 44 is released and the brake 54 is brought to its engagedposition while the underdrive clutch 30 is maintained engaged, the firstring gear 50 and the second sun gear 52 are stopped in rotation andserve respectively as reaction elements. Thus, the driving force istransmitted to the output gear 64 through the first sun gear 46, thefirst carrier 38, the second carrier 42, the second ring gear 60 and theoutput shaft 62, so that shifting into the 2nd gear ratio is achieved.

As the 2-4 brake 54 is released and the overdrive clutch 28 is broughtto its engaged position while the underdrive clutch 30 is maintainedengaged, the first sun gear 46 and the first carrier 38 rotate together,so that the entire component parts of the first gear unit 36 rotatetogether. Accordingly, the second sun gear 52 and the second carrier 42rotate together, so that the entire component parts of the second gearunit 40 also rotate together. Thus, shifting into the 3rd gear ratio isachieved in which the input shaft 26 and the output shaft 64 are broughtto the same rotational speed.

As the underdrive clutch 30 is released and the 2-4 brake 54 is broughtto its engaged position while the overdrive clutch 28 is maintainedengaged, the second sun gear 52 serves as a reaction element, so thatthe driving force is transmitted to the second carrier 42 through thefirst intermediate shaft 34. The second pinion gear 58 moves around thesecond sun gear 52 while rotating about an axis of the second piniongear 58, whereby the driving force is further transmitted to the outputgear 64 through the output shaft 62. Thus, shifting into the overdrive4th gear ratio is achieved in which rotation of the output gear 64 isfaster than that of the input shaft 26.

As the overdrive clutch 28 and the 2-4 brake 54 are released fromengagement and the reverse clutch 32 and the low/reverse brake 44 arebrought to their respective engaged positions, the second carrier 42serves as a reaction element, so that the driving force is transmittedto the output gear 64 through the second intermediate shaft 48, thesecond sun gear 52, the second pinion gear 58, the second ring gear 60and the output shaft 62. Thus, shifting into the reverse gear ratio isachieved.

The speed change gear illustrated in FIG. 1 has associated therewith ahydraulic control system for achieving shifting into the various gearratios indicated in the table 1. The arrangement and operation of thehydraulic control system will next be described with reference to FIG.2.

The hydraulic control system shown in FIG. 2 is designed to controlhydraulic pressure supplied to the torque converter 12, and hydraulicpressure supplied to each of the clutches 28, 30 and 32 and the brakes44 and 54 of the speed change gear shown in FIG. 1, in accordance withrunning conditions of a vehicle. The hydraulic control system comprisesprimary component parts including an oil pump 86, a pressure regulatingvalve 100, a torque converter control valve 200, a manually operatedvalve 300, a first solenoid valve 400A serving as first shift means, asecond solenoid valve 400B serving as second shift means, a thirdsolenoid valve 400C serving as fourth shift means, a fourth solenoidvalve 400D serving as third shift means, a line pressure switching valve500, a first fail-safe valve 600 serving as first valve means, and asecond fail-safe valve 700 serving as second valve means. Thesecomponent parts are connected to each other by hydraulic lines.

The oil pump 86 is adapted to draw hydraulic fluid reserved in an oilpan 80 through a filter 82 and a hydraulic line 84.. The drawn hydraulicfluid is pressurized by the oil pump 86 and is discharged into ahydraulic line 88.

The pressure regulating valve 100 is designed to regulate the hydraulicpressure or line pressure in the hydraulic line 88 to desirable valuescorresponding respectively to the gear ratios. The pressure regulatingvalve 100 is composed of a spool 138 formed with six lands 106, 112,118, 124, 130 and 136, and a spring 140 which is abutted against theland 136 for biasing the spool 138 to the right as viewed in FIG. 2. Theland 106 has a pair of pressure receiving faces 102 and 104. The land112 has a pressure receiving face 108 confronted with the pressurereceiving face 104, and a pressure receiving face 110. The land 118 hasa pressure receiving face 114 confronted with the pressure receivingface 110, and a pressure receiving face 116. The land 124 has a pressurereceiving face 120 confronted with the pressure receiving face 116, anda pressure receiving face 122. The land 130 has a pressure receivingface 126 substantially confronted with the pressure receiving face 122,and a pressure receiving face 128. The land 136 has a pressure receivingface 132 confronted with the pressure receiving face 128. The pressurereceiving face 108 has a pressure receiving area larger than that of thepressure receiving face 104. The pressure receiving face 114 has apressure receiving area larger than that of the pressure receiving face110. The pressure receiving face 120 has a pressure receiving arealarger than that of the pressure receiving face 116. The pressurereceiving faces 122 and 126 have the same pressure receiving area. Thepressure receiving faces 128 and 132 have the same pressure receivingarea.

A hydraulic line 144 having provided therein an orifice 142 alwayscommunicates with a pressure space facing the pressure receiving face102. The hydraulic line 88 always communicates with a space definedbetween the pressure receiving faces 104 and 108, through a hydraulicline 148 having provided therein an orifice 146. A hydraulic line 152having provided therein an orifice 150 always communicates with a spacedefined between the pressure receiving faces 110 and 114. A hydraulicline 156 having provided therein an orifice 154 always communicates witha space defined between the pressure receiving faces 116 and 120. Thehydraulic line 88 always communicates with a space defined between thepressure receiving faces 128 and 132, through a hydraulic line 158. Thehydraulic line 84 always communicates with a space defined between thepressure receiving faces 122 and 126, through a hydraulic line 166. Whenthe spool 138 moves to the left as viewed in FIG. 2, the space definedbetween the pressure receiving faces 128 and 132 is brought intocommunication with a hydraulic line 164 which communicates with adownstream side of an orifice 162 provided in a hydraulic line 160.

The torque converter control valve 200 is composed of a spool 214 formedwith two lands 206 and 212, and a spring 216 abutted against the land212 for biasing the spool 214 to the right as viewed in FIG. 2. The land206 has a pair of pressure receiving faces 202 and 204. The land 212 hasa pressure receiving face 208 which is confronted with the pressurereceiving face 204 and which is the same in pressure receiving area asthe pressure receiving face 204. The hydraulic pressure in the hydraulicline 88 regulated in pressure by the pressure regulating valve 100 isapplied to the pressure receiving face 202 through the hydraulic line160, a hydraulic line 168 and a hydraulic line 170 having providedtherein an orifice 172. The spool 214 moves to a position where thepressure acting upon the pressure receiving face 202 and the biasingforce of the spring 216 balance with each other, thereby regulating thehydraulic fluid discharged to the hydraulic line 168 to a predeterminedpressure. The hydraulic fluid regulated to the predetermined pressure issupplied to the torque converter 12 through the hydraulic line 168. Thehydraulic fluid discharged from the torque converter 12 is supplied tovarious lubricating sections of the transmission through a hydraulicline 174.

The manually operated valve 300 comprises a spool 302 movable to aselected one of three positions including a reverse position R, aneutral or parking position N/P and a drive position D. The spool 302 isformed with four lands 304, 306, 308 and 310 and a coupling member 303which is mechanically or electrically connectable to a conventionallyknown selector lever (not shown) arranged within the vehicle compartmentfor setting the spool 302 to a desired position. The selector lever isusually movable among a P position for parking, an R position forreverse, an N position for stoppage, a D4 position where shifting ismade possible among 1st through 4th forward gear ratios, a D3 positionwhere shifting is made possible among 1st through 3rd forward gearratios, a D2 position where shifting into gear ratios equal to or higherthan the 3rd gear ratio is prohibited, and an L position where shiftinginto gear ratios equal to or higher than the 2nd gear ratio isprohibited. As the selector lever is operated to select any one of theD4, D3, D2 and L positions, the spool 302 is moved to the D position. Inthe D position, the hydraulic line 88 is brought into communication witha hydraulic line 314 and with a hydraulic line 316 leading to thesolenoid valves, through a space defined between the lands 304 and 306.Also, in the D position, the hydraulic line 144 is brought intocommunication with an exhaust line 320 leading to an exhaust port 318,through a space between the lands 308 and 310, a hydraulic line 322, anda hydraulic pressure chamber 323 on the right-hand side of the land 304.Communication of the hydraulic line 88 with the hydraulic line 316through the manually operated valve 300 causes the hydraulic pressure inthe hydraulic line 88 to be supplied to the clutches 28 and 30 and thebrakes 44 and 54, in accordance with combinations in energization anddeenergization of the first solenoid valve 400A, the second solenoidvalve 400B, the third solenoid valve 400C and the fourth solenoid valve400D, subsequently to be described in detail. In this manner, the speedchange gear illustrated in FIG. 1 is switched to one of the forward gearratios which corresponds to a selected position of the selector leverand to the running conditions of the vehicle.

As the selector lever is selectively moved to the P position or the Nposition, the spool 302 is moved to the N/P position. In the N/Pposition, the hydraulic line 88 is brought into communication with thehydraulic line 144 through a hydraulic line 324 and the space betweenthe lands 308 and 310. The hydraulic line 88 is also brought intocommunication with the hydraulic line 314 through the space between thelands 304 and 306. A hydraulic line 328 having provided therein anorifice 326 and connected to the reverse clutch 32 is brought intocommunication with the exhaust line 320 through a space between thelands 306 and 308, the hydraulic line 322 and the hydraulic chamber 323.Further, the hydraulic line 316 is also brought into communication withthe exhaust line 320. Thus, the neutral state is achieved.

As the selector lever is selectively moved to the R position, the spool302 is moved to the R position. In the R position, the hydraulic line 88is brought into communication with the hydraulic lines 314 and 328through the space between the lands 304 and 306. In addition, thehydraulic line 144 is brought into communication with the exhaust line320 through the space between the lands 306 and 308, the hydraulic line322 and the hydraulic pressure chamber 323. The hydraulic line 316 isalso brought into communication with the exhaust line 320. Thus,shifting into the reverse gear ratio is achieved in the speed changegears, subsequently to be described.

The first solenoid valve 400A is a three-way valve of normally opentype. The first solenoid valve 400A has incorporated therein a coil402a, a valve member 404a and a spring 406a biasing the valve member404a toward its open position. In a state in which the coil 402a isdeenergized, the valve member 404a intercepts communication between theexhaust line 320 and a hydraulic line 410 having provided therein anorifice 408 and connected to the first fail-safe valve 600 subsequentlyto be described in detail. In addition, the valve member 404a brings thehydraulic line 410 into communication with a hydraulic line 414 intowhich the hydraulic pressure in the hydraulic line 316 or 328 isintroduced through a check valve 412. In a state in which the coil 402ais energized, the valve member 404a intercepts communication between thehydraulic lines 410 and 414, and brings the hydraulic line 410 intocommunication with the exhaust line 320.

The second solenoid valve 400B is a three-way valve of normally closedtype. The second solenoid valve 400B has incorporated therein a coil402b, a valve member 404b, and a spring 406b biasing the valve member404b toward its closed position. In a state in which the coil 402b isdeenergized, the valve member 404b intercepts communication between thehydraulic line 316 and a hydraulic line 416 having provided therein anorifice 415 and connected to the second fail-safe valve 700 subsequentlyto be described in detail. In addition, the valve member 404b brings thehydraulic line 416 into communication with the exhaust line 320. In astate in which the coil 402b is energized, the valve member 404bintercepts communication between the exhaust line 320 and the hydraulicline 416, and brings the hydraulic line 416 into communication with thehydraulic line 316.

Each of the third and fourth solenoid valves 400C and 400D is athree-way valve of normally open type similar to the first solenoidvalve 400A. The third and fourth solenoid valves 400C and 400D haveincorporated respectively therein coils 402c and 402d, valve members404c and 404d, and springs 406c and 406d biasing the respective valvemembers 404c and 404d toward their respective open positions. In a statein which the coil 402c of the third solenoid valve 400C is deenergized,the valve member 404c intercepts communication between the exhaust line320 and a hydraulic line 422 having provided therein an orifice 418 andconnected to the underdrive clutch 30. In addition, the valve member404c brings the hydraulic line 422 into communication with the hydraulicline 316. In a state in which the coil 402c is energized, the valvemember 404c brings the exhaust line 320 into communication with thehydraulic line 422, and intercepts communication between the hydraulicline 422 and the hydraulic line 316. On the other hand, in a state inwhich the coil 402d of the fourth solenoid valve 400D is deenergized,the valve member 404d intercepts communication between the exhaust line320 and a hydraulic line 424 having provided therein an orifice 420 andconnected to the overdrive clutch 28. In addition, the valve member 404dbrings the hydraulic line 424 into communication with the hydraulic line316. In a state in which the coil 402d is energized, the valve member404d brings the exhaust line 320 into communication with the hydraulicline 424, and intercepts communication between the hydraulic line 424and the hydraulic line 316.

The first through fourth solenoid valves 400A through 400D are operativein response to electric signals from an electronic control system (notshown). The below table 2 indicates the relationship between the gearratios and combinations in "ON" (energization) and "OFF"(deenergization) of the first through fourth solenoid valves 400A, 400B,400C and 400D. In the table 2, the symbol "-" represents that thesolenoid valve may be in either one of the ON and OFF positions.

                  TABLE 2                                                         ______________________________________                                                 First      Second   Third    Fourth                                           Solenoid   Solenoid Solenoid Solenoid                                Gear     Valve      Valve    Valve    Valve                                   Speed    400 A      400 B    400 C    400 D                                   ______________________________________                                        1st      OFF        OFF      OFF      ON                                      2nd      ON         ON       OFF      ON                                      3rd      ON         OFF      OFF      OFF                                     4th      ON         ON       ON       OFF                                     Reverse  OFF        --       --       --                                      ______________________________________                                    

The line pressure switching valve 500 is provided for switching the linepressure to values corresponding respectively to the gear ratios, and iscomposed of a spool 518 and a spring 520. The spool 518 is formed with aland 508 having a pair of pressure receiving faces 504 and 506, and aland 516 having a pair of pressure receiving faces 510 and 514. Thepressure receiving face 510 is confronted with the pressure receivingface 506 and has a pressure receiving area the same as that of thepressure receiving face 506. The hydraulic pressure in the hydraulicline 314 having provided therein an orifice 502 is applied to thepressure receiving face 504. The hydraulic pressure in a hydraulic line512a branching from the hydraulic line is applied to the pressurereceiving face 514. The spring 520 is abutted against the pressurereceiving face 514 to bias the spool 518 to the right as viewed in FIG.2.

When the hydraulic pressure is supplied to the hydraulic line 314, butis not supplied to the hydraulic line 512a, the pressure acting upon thepressure receiving face 504 overcomes the biasing force of the spring520 to displace the spool 518 to the left as viewed in FIG. 2. Thus,communication between the hydraulic line 156 and the hydraulic line 512ais intercepted, and the hydraulic line 156 is brought into communicationwith an exhaust port through a space defined between the lands 508 and516. When no hydraulic pressure is supplied to both the hydraulic lines512a and 314, or when the hydraulic pressure is supplied only to thehydraulic line 512a, the spool 518 is displaced to the right as viewedin FIG. 2 under the biasing force of the spring 520 or under the biasingforce thereof and the pressure acting upon the pressure receiving face514, to bring the hydraulic line 156 into communication with thehydraulic line 512a through a hydraulic pressure chamber on theleft-hand side of the land 516. The hydraulic line 512a has providedtherein an orifice 522. An orifice 524 is provided in another hydraulicline 512b branching from the hydraulic line 424. The hydraulic line 512bis connected to the first fail-safe valve 600 and the second fail-safevalve 700 subsequently to be described in detail.

The first fail-safe valve 600 has a spool 636 formed with five lands610, 616, 622, 628 and 634. The land 610 has a pair of pressurereceiving faces 606 and 608. The land 616 has a pressure receiving face612 confronted with the pressure receiving face 608 and has a pressurereceiving area larger than that of the pressure receiving face 608. Theland 616 also has a pressure receiving face 614. The land 622 has apressure receiving face 618 confronted with the pressure receiving face614 and has a pressure receiving area smaller than that of the pressurereceiving face 614. The land 622 also has a pressure receiving face 620.The land 628 has a pressure receiving face 624 confronted with thepressure receiving face 620 and has a pressure receiving area the sameas that of the pressure receiving face 620. The land 628 also has apressure receiving face 626. The land 634 has a pressure receiving face630 confronted with the pressure receiving face 626 and has a pressurereceiving area smaller than that of the pressure receiving face 626. Theland 634 also has a pressure receiving face 632. Hydraulic pressure in ahydraulic line 604 having provided therein an orifice 602 and branchingfrom the hydraulic line 314 is applied to the pressure receiving face606. The hydraulic pressure in the hydraulic line 512b is applied to thepressure receiving face 632. The hydraulic pressure in the hydraulicline 410 is applied to the pressure receiving faces 614 and 618.Hydraulic pressure in a hydraulic line 644 having provided therein anorifice 642 is applied to the pressure receiving faces 626 and 630. Thehydraulic line 644 branches from a hydraulic line 640 for supplying thehydraulic pressure to the 2-4 brake 54.

Only when the hydraulic pressure in the hydraulic line 604 is applied tothe pressure receiving face 606 and the hydraulic pressure in thehydraulic line 410 is applied to the pressure receiving faces 614 and618, the spool 636 is retained at the left-hand end position in FIG. 2so that the hydraulic line 410 is maintained in communication with ahydraulic line 638 for supplying the hydraulic pressure to thelow/reverse brake 44. On the other hand, when the hydraulic pressure inthe hydraulic line 604 is applied to the pressure receiving face 606 andthe hydraulic pressure in the hydraulic line 410 is applied to thepressure receiving faces 614 and 618 and, in addition thereto, when thehydraulic pressure is introduced into at least one of the hydraulic line644 and the hydraulic line 512b (which communicates with the hydraulicline 424 for supplying the hydraulic pressure to the overdrive clutch28) so that the hydraulic pressure is applied to at least one of thepressure receiving faces 626 and 632, the spool 636 is displaced to theright as viewed in FIG. 2 to bring the hydraulic line 638 intocommunication with the exhaust line 320 through a space between thelands 622 and 628 thereby releasing the low/reverse brake 44 in amoment. A space between the lands 610 and 616 communicates with anexhaust port.

As will be appreciated from the above, the pressure receiving faces 614and 618 serve as detecting means for detecting the hydraulic pressuresupplied from the hydraulic line 410 to the low/reverse brake 44. Thepressure receiving faces 626 and 630 serve as detecting means fordetecting the hydraulic pressure supplied from the hydraulic lines 416and 640 to the 2-4 brake 54. The pressure receiving face 632 serves asdetecting means for detecting the hydraulic pressure supplied from thehydraulic line 424 to the overdrive clutch 28. In addition, theabove-described various pressure receiving faces also serve as switchingmeans for switching the positions of the spool 636.

The second fail-safe valve 700 has a spool 732 formed with five lands706, 712, 718, 724 and 730. The land 706 has a pair of pressurereceiving faces 702 and 704. The land 712 has a pressure receiving face708 confronted with the pressure receiving face 704 and has a pressurereceiving area larger than that of the pressure receiving face 704. Theland 712 also has a pressure receiving face 710. The land 718 has apressure receiving face 714 confronted with the pressure receiving face710 and has a pressure receiving area smaller than that of the pressurereceiving face 710. The land 718 also has a pressure receiving face 716.The land 724 has a pressure receiving face 720 confronted with thepressure receiving face 716 and has a pressure receiving area the sameas that of the pressure receiving face 716. The land 724 also has apressure receiving face 722. The land 730 has a pressure receiving face726 confronted with the pressure receiving face 722 and has a pressurereceiving area smaller than that of the pressure receiving face 722. Theland 730 also has a pressure receiving face 728. The hydraulic pressurein the hydraulic line 604 is applied to the pressure receiving faces 704and 708. The hydraulic pressure in the hydraulic line 416 is applied tothe pressure receiving faces 710 and 714. Hydraulic pressure in ahydraulic line 736 having provided therein an orifice 734 and branchingfrom the hydraulic line 422 is applied to the pressure receiving faces722 and 726. The hydraulic pressure in the hydraulic line 512b isapplied to the pressure receiving face 728.

When the hydraulic pressure in the hydraulic line 604 is introduced tothe pressure receiving faces 704 and 708 and the hydraulic pressure inthe hydraulic line 416 is introduced to the pressure receiving faces 710and 714 and, simultaneously, when the hydraulic pressure in only one ofthe hydraulic lines 512b and the hydraulic line 736 is introduced to thepressure receiving face 728 or the pressure receiving faces 722 and 726,the difference in area among the pressure receiving faces having appliedthereto their respective hydraulic pressures causes the spool 732 to bemaintained at the left-hand end position in FIG. 2, thereby maintainingthe hydraulic line 416 in communication with the hydraulic line 640 forsupplying the hydraulic pressure to the 2-4 brake 54, through a spacedefined between the lands 712 and 718. As the hydraulic pressure in thehydraulic line 604 is introduced to the pressure receiving faces 704 and708 and the hydraulic pressure in the hydraulic line 416 is introducedto the pressure receiving faces 710 and 714 and, simultaneously, as boththe hydraulic pressure in the hydraulic line 512b and the hydraulicpressure in the hydraulic line 736 are introduced respectively to thepressure receiving face 728 and the pressure receiving faces 722 and726, the spool 732 is displaced to the right as viewed in FIG. 2 tobring the hydraulic line 640 into communication with the exhaust line320 through a space between the lands 718 and 724, thereby releasing the2-4 brake 54 in a moment. A hydraulic pressure chamber on the right-handside of the land 706 communicates with an exhaust port.

As will be appreciated from the above, the pressure receiving faces 710and 714 serve as detecting means for detecting the hydraulic pressuresupplied from the hydraulic line 416 to the 2-4 brake 54. The pressurereceiving faces 722 and 726 serve as detecting means for detecting thehydraulic pressure supplied from the hydraulic line 422 to theunderdrive clutch 30. The pressure receiving face 728 serves asdetecting means for detecting the hydraulic pressure supplied from thehydraulic line 424 to the overdrive clutch 28. In addition, theabove-described various pressure receiving faces also serve as switchingmeans for switching the positions of the spool 732.

The first and second fail-safe valves 600 and 700 serve as follows. Thatis, even if one or more solenoid valves malfunction when shifting to acertain forward gear ratio has been achieved, the first and secondfail-safe valves 600 and 700 prevent occurrence of such a situation thatthree or more engaging means are simultaneously brought to theirrespective engaged positions causing the transmission to be locked, andthe first and second fail-safe vales 600 and 700 forcibly shift theforward gear ratio to the 2nd, 3rd or 4th gear ratio to enable thevehicle to run.

The operation of the hydraulic control system constructed as above willnext be described.

As a driver moves the selector lever arranged within the vehiclecompartment, to select one of the P and N positions, the spool 302 ofthe manually operated valve 300 mechanically or electrically connectedto the selector lever is also moved to the P/N position in interlockedrelation to the selector lever. As the engine is started, the hydraulicfluid pressurized by the oil pump 86 is discharged into the hydraulicline 88 so that the hydraulic pressure is generated in the hydraulicline 88. The hydraulic pressure in the hydraulic line 88 is applied tothe pressure receiving faces 104 and 108 of the pressure regulatingvalve 100 through the hydraulic line 148, and is also applied to thepressure receiving face 102 of the pressure regulating valve 100 throughthe hydraulic line 324, the space between the lands 308 and 310 of themanually operated valve 300, and the hydraulic line 144. The spool 138of the pressure regulating valve 100 is stabilized at a position wherethe pressures acting upon the respective pressure receiving facesbalance with the biasing force of the spring 140. A part of thehydraulic pressure introduced into the space between the lands 130 and136 through the hydraulic line 158 is discharged to the hydraulic lines164 and 166, so that the hydraulic pressure in the hydraulic line 88 isregulated to a predetermined lowest pressure (hereinafter, referred toas "first line pressure"). The hydraulic fluid in the hydraulic lines 88and 164 is introduced into the torque converter control valve 200through the hydraulic line 160. The above-described hydraulic pressurecontrol operation of the torque converter control valve 200 causes thehydraulic fluid of a predetermined pressure to be supplied to the torqueconverter 12 through the hydraulic line 168. Further, the hydraulicpressure in the hydraulic line 88 is introduced into a right-hand endhydraulic pressure chamber of the line pressure switching valve 500 andinto the hydraulic line 604 through the space between the lands 304 and306 of the manually operated valve 300 and through the hydraulic line314. The hydraulic pressure introduced into the right-hand end hydraulicpressure chamber of the line pressure switching valve 500 is applied tothe pressure receiving face 504, and overcomes the biasing force of thespring 520 to displace the spool 518 to the left-hand end position inFIG. 2, thereby bringing the hydraulic line 156 into communication withthe exhaust port. The hydraulic pressure introduced into the hydraulicline 604 is introduced into a right-hand end hydraulic pressure chamberof the first fail-safe valve 600 and into a space between the lands 712and 706 of the second fail-safe valve 700. This displaces the spool 636and 732 of the respective fail-safe valves 600 and 700 to theirrespective left-hand end positions in FIG. 2, thereby bringing thehydraulic line 410 into communication with the hydraulic line 638 andbringing the hydraulic line 416 into communication with the hydraulicline 640. In the meantime, all of the first through fourth solenoidvalves 400A, 400B, 400C and 400D are maintained in their respective OFFor deenergized positions.

As the driver operates the selector lever to select the D4 position, themanually operated valve 300 is moved to the D position to cause the land310 to intercept the hydraulic line 324. This brings the hydraulic line144 into communication with the hydraulic line 322 so that the hydraulicpressure in the right-hand end hydraulic pressure chamber of thepressure regulating valve 100 is discharged through the hydraulic line322 and the exhaust line 320. The hydraulic line 88 is further broughtinto communication also with the hydraulic line 316 through the spacebetween the lands 304 and 306 of the manually operated valve 300 so thatthe hydraulic pressure in the hydraulic line 316 is introduced into thespace between the lands 112 and 118 of the pressure regulating valve 100through the hydraulic line 152. The spool 138 is stabilized at aposition where the forces acting upon the respective pressure receivingfaces 110 and 114 and the respective pressure receiving faces 104 and108 balance with the biasing force of the spring 140, to discharge apart of the hydraulic pressure in the hydraulic line 158 from thehydraulic lines 164 and 166, thereby regulating the hydraulic pressurein the hydraulic line 88 to a predetermined pressure (hereinafter,referred to as "second line pressure"), e.g., 10 kg/cm². The hydraulicpressure in the hydraulic line 316 is introduced into the second throughfourth solenoid valves 400B, 400C and 400D, and is also introduced intothe first solenoid valve 400A through the check valve 412 and thehydraulic line 414.

The hydraulic line 314 is maintained in communication with the hydraulicline 88, similarly to the case of the above-described N/P position.

As the selector lever is moved to the D4 position, signals are outputtedfrom the electronic control system (not shown) to maintain the firstthrough third solenoid valves 400A, 400B and 400C in their respectivedeenergized positions and to move the fourth solenoid valve 400D fromthe deenergized position to the energized position as indicated in thetable 2, in order to bring the low/reverse brake 44 and the underdriveclutch 30 to their respective engaged positions to achieve shifting tothe 1st gear ratio as indicated in the table 1. Only the first and thirdsolenoid valves 400A and 400C are in their respective open positionswhere the hydraulic lines 414 and 410 are brought into communicationwith each other and the hydraulic lines 316 and 422 are brought intocommunication with each other.

The hydraulic pressure introduced into the hydraulic line 410 isintroduced into the low/reverse brake 44 through a space between thelands 616 and 622 of the first fail-safe valve 600 and through thehydraulic line 638. The hydraulic pressure introduced into the hydraulicline 422 is introduced into the underdrive clutch 30 and into a spacebetween the lands 724 and 730 of the second fail-safe valve 700 throughthe hydraulic line 736. On this occasion, if both the engaging means 44and 30 are abruptly brought to their respective engaged positions withthe relatively high second line pressure, shifting shocks might occur.By this reason, the first solenoid valve 400A is first energized at apredetermined duty ratio just before the low/reverse brake 44 is broughtto its engaged position, and the third solenoid valve 400C is firstenergized at a predetermined duty ratio just before the underdriveclutch 30 is brought to its engaged position. Then, the duty ratios ofthe respective first and third solenoid valves 400A and 400C aregradually reduced, and the first and third solenoid valves 400A and 400Care finally deenergized. In this manner, the hydraulic pressures in therespective hydraulic lines 638 and 422 rise gradually. This makes itpossible to reduce the above-mentioned shifting shocks.

As the vehicle starts to run, and as the electronic control system isoperative in response to a throttle valve opening degree signal, avehicle speed signal and the like to command to shift up into the 2ndgear ratio, signals are outputted from the electronic control system toenergize the first, second and fourth solenoid valves 400A, 400B and400D and to deenergize the third solenoid valve 400C, to bring the 2-4brake 54 and the underdrive clutch 30 to their respective engagedpositions as indicated in the table 1. Since the first solenoid valve400A is switched from the deenergized position to the energizedposition, the hydraulic line 410 is brought into communication with theexhaust line 320 to discharge the hydraulic pressure from the hydraulicline 410, thereby releasing the low/reverse brake 44.

Since the second solenoid valve 400B is switched from the deenergizedposition to the energized position, the hydraulic line 316 is broughtinto communication with the hydraulic line 416. Thus, the hydraulicpressure introduced into the hydraulic line 416 is introduced into the2-4 brake 54 through the space between the lands 712 and 718 of thesecond fail-safe valve 700 and through the hydraulic line 640, to bringthe brake 54 to its engaged position. The hydraulic pressure in thehydraulic line 640 is also introduced into a space between the lands 628and 634 of the first fail-safe valve 600 through the hydraulic line 644.Since, however, the hydraulic pressure having acted upon the spacebetween the lands 616 and 622 has been reduced, the resultant force, tothe right in FIG. 2, of the hydraulic pressure introduced into the spacebetween the lands 616 and 622 and acting upon the pressure receivingfaces 626 and 630 cannot overcome the biasing force to the left in FIG.2 due to the second line pressure acting upon the pressure receivingface 606 of the land 610. Thus, the spool 636 is maintained at theposition on the left-hand side in FIG. 2. Like the above-mentioned firstand third solenoid valves 400A and 400C, if the second solenoid valve400B is abruptly energized, the 2-4 brake 54 is suddenly brought to itsengaged position, so that shifting shocks might occur. Accordingly, theduty ratio of the solenoid valve 400B is gradually varied to graduallyraise the hydraulic pressure in the hydraulic line 416 such that thehydraulic pressure in the hydraulic line 416 finally reaches the secondline pressure. This makes it possible to reduce the shifting shocks.

Since the third solenoid valve 400C is maintained deenergized, thehydraulic line 316 communicates with the hydraulic line 422 so that theunderdrive clutch 30 is maintained engaged, like the case of the 1stgear ratio.

Since the fourth solenoid valve 400D is maintained energized to bringthe hydraulic line 424 into communication with the exhaust line 320, sothat the overdrive clutch 28 is maintained released.

As the vehicle speed further increases, and as the electronic controlsystem commands to shift up from the 2nd gear ratio into the 3rd gearratio, signals are outputted from the electronic control system toenergize the first solenoid valve 400A and to deenergize the secondthrough fourth solenoid valves 400B, 400C and 400D, to bring theunderdrive clutch 30 and the overdrive clutch 28 to their respectiveengaged positions as indicated in the table 1.

Since the first solenoid valve 400A is maintained energized, thehydraulic line 410 is brought into communication with the exhaust line320, like the case of the 2nd gear ratio.

Since the second solenoid valve 400B is switched from the energizedposition to the deenergized position, the hydraulic line 416 is broughtinto communication with the exhaust 320, thereby releasing the 2-4 brake54.

Since the third solenoid valve 400C is maintained deenergized, thehydraulic line 422 is retained in communication with the hydraulic line316, so that the underdrive clutch 30 is maintained engaged.

Since the fourth solenoid valve 400D is switched from the energizedposition to the deenergized position, the hydraulic line 316 is broughtinto communication with the hydraulic line 424. The hydraulic pressureintroduced into the hydraulic line 424 is introduced into the overdriveclutch 28 to bring the same to its engaged position. The hydraulicpressure in the hydraulic line 424 is further introduced into theleft-hand end hydraulic pressure chamber of the line pressure switchingvalve 500 through the hydraulic line 512a, and into the respectiveleft-hand end hydraulic pressure chambers of the first and secondfail-safe valve 600 and 700 through the hydraulic line 512b. Thehydraulic pressure introduced into the left-hand end hydraulic pressurechamber of the line pressure switching valve 500 is applied to thepressure receiving face 514 and cooperates with the biasing force of thespring 520 to overcome the hydraulic pressure acting on the pressurereceiving face 504 of the land 508, thereby displacing the spool 518 tothe right in FIG. 2 to bring the hydraulic line 512a into communicationwith the hydraulic line 156. The hydraulic pressure introduced from thehydraulic line 512a into the hydraulic line 156 is introduced into thespace between the lands 118 and 124 of the pressure regulating valve 100and is applied to the pressure receiving faces 116 and 120. Thus, thespool 138 of the pressure regulating valve 100 is stabilized at aposition where the forces acting respectively upon the pressurereceiving faces 116 and 120, the pressure receiving faces 110 and 114and the pressure receiving faces 104 and 108 balance with the biasingforce of the spring 140. In this stabilized position, a part of thehydraulic pressure in the hydraulic line 158 is discharged from thehydraulic lines 164 and 166, so that the hydraulic pressure in thehydraulic line 88 is regulated to a predetermined pressure (hereinafter,referred to as "third line pressure") which is lower than the secondline pressure, but is higher than the first line pressure.

The hydraulic pressure introduced into the left-hand end hydraulicpressure chamber of the first fail-safe valve 600 is applied to thepressure receiving face 632 of the land 634, but cannot overcome thehydraulic pressure introduced into the right-hand end hydraulic pressurechamber of the valve 600 through the hydraulic line 604 and acting uponthe pressure receiving face 606, because of the difference in areabetween the pressure receiving faces 632 and 606. Accordingly, the spool636 is maintained at the position on the left-hand side in FIG. 2.

The hydraulic pressure introduced into the left-hand end hydraulicpressure chamber of the second fail-safe valve 700 is applied to thepressure receiving face 728 of the land 730. The hydraulic pressureacting upon the pressure receiving face 728 cooperates with thehydraulic pressure introduced into the space between the lands 724 and730 through the hydraulic line 736 and acting upon the pressurereceiving faces 722 and 726, to bias the spool 732 to the right asviewed in FIG. 2. However, the biasing force to the right is lower thanthe biasing force to the left due to the hydraulic pressure which isintroduced into the space between the lands 706 and 712 through thehydraulic line 604 and which acts upon the pressure receiving faces 704and 708. Accordingly, the spool 732 is maintained at the position on theleft-hand side in FIG. 2.

As the vehicle speed further increases, and as the electronic controlsystem commands to shift up from the 3rd gear ratio into the 4th gearratio, signals are outputted from the electronic control system toenergize the first through third solenoid valves 400A, 400B and 400C andto deenergize the fourth solenoid valve 400D, to bring the 2-4 brake 54and the overdrive clutch 28 to their respective engaged positions asindicated in the table 1. Since the first solenoid valve 400A ismaintained energized, the hydraulic line 410 is brought intocommunication with the exhaust line 320, like the case of the 3rd gearratio.

Since the second solenoid valve 400B is switched from the deenergizedposition to the energized position, the hydraulic line 316 is broughtinto communication with the hydraulic line 416. Accordingly, thehydraulic pressure in the hydraulic line 416 is introduced into the 2-4brake 54 through the space between the lands 712 and 718 of the secondfail-safe valve 700 and through the hydraulic line 640, thereby bringingthe 2-4 brake 54 to its engaged position. At the same time, thehydraulic pressure in the hydraulic line 416 is introduced into thespace between the lands 628 and 634 of the first fail-safe valve 600through the hydraulic line 644, and is applied to the pressure receivingfaces 626 and 630.

Since the third solenoid valve 400C is switched from the deenergizedposition to the energized position, the hydraulic line 422 isintercepted from communication with the hydraulic line 316, but isbrought into communication with the exhaust line 320, so that thehydraulic pressure in the hydraulic line 736 is discharged and,simultaneously, the underdrive clutch 30 is released.

Since the fourth solenoid valve 400D is maintained deenergized so thatthe hydraulic line 424 is retained in communication with the hydraulicline 316, the overdrive clutch 28 is maintained engaged. Thus, thehydraulic pressure continues to be introduced into the left-hand endhydraulic chamber of the line pressure switching valve 500 to maintainthe hydraulic pressure in the hydraulic line 88 at the second linepressure. The hydraulic pressure also continues to be introduced intothe left-hand end hydraulic pressure chamber of the first fail-safevalve 600 and into the left-hand end hydraulic pressure chamber of thesecond fail-safe valve 700.

In the state in which shifting into the 4th gear ratio has beenachieved, the hydraulic pressure is introduced into the left-hand endhydraulic pressure chamber of the first fail-safe valve 600 and into thespace between the lands 628 and 634. Accordingly, because the sum of thepressure receiving areas of the respective pressure receiving faces 632,626 and 630 is larger than that of the pressure receiving face 606 underconsideration of the hydraulic pressure acting direction, the hydraulicpressures acting respectively upon the pressure receiving faces 632, 626and 630 overcome the hydraulic pressure acting upon the pressurereceiving face 606, to displace the spool 636 to the right in FIG. 2,thereby bringing the hydraulic line 638 into communication with theexhaust line 320.

The operation of shifting-up from the 1st gear ratio to the 2nd gearratio, from the 2nd gear ratio to the 3rd gear ratio and from the 3rdgear ratio to the 4th gear ratio has been described above. The operationof shifting-down from the 4th gear ratio to the 3rd gear ratio, from the3rd gear ratio to the 2nd gear ratio and from the 2nd gear ratio to the1st gear ratio is merely carried out in the steps of procedurecompletely reverse to those described above, and the description of theshifting-down will therefore be omitted.

In addition, when the selector lever is moved to one of the D3, D2 and Lpositions, a speed range corresponding to the selected position ismerely determined by the commands from the electronic control system.The manually operated valve 300 is maintained at the D position, and nochange occurs in the hydraulic circuit. Thus, the description of thecase where the selector lever is moved to one of the D3, D2 and Lpositions will therefore be omitted.

As the driver moves the selector lever to the R position, the spool 302of the manually operated valve 300 is also moved to the R position, sothat the hydraulic line 144 is brought into communication with theexhaust line 320 through the space between the lands 306 and 308, thehydraulic line 322 and the right-hand end hydraulic pressure chamber 323of the manually operated valve 300. In addition, the hydraulic line 316is also brought into communication with the exhaust line 320 through theright-hand end hydraulic pressure chamber 323. Accordingly, introducedinto the pressure regulating valve 100 is only the hydraulic pressurewhich is introduced into the space between the lands 106 and 112 of thepressure regulating valve 100 and which is applied to the pressurereceiving faces 104 and 108. This stabilizes the spool 138 at a positionwhere the hydraulic pressure acting upon the pressure receiving faces104 and 108 balances with the biasing force of the spring 140, so thatthe hydraulic pressure in the hydraulic line 88 is regulated to apredetermined highest pressure (hereinafter, referred to as "forth linepressure"), e.g., 16 kg cm². Moreover, the hydraulic line 88 is broughtinto communication with the hydraulic lines 314 and 328 through thespace between the lands 304 and 306 of the manually operated valve 300.The hydraulic pressure introduced into the hydraulic line 314 isintroduced into the line pressure switching valve 500, and into thefirst and second fail-safe valves 600 and 700 through the hydraulic line604, like the case where the shifting into the forward gear ratios isachieved. The hydraulic pressure introduced into the hydraulic line 328is introduced into the reverse clutch 32 to bring the same to itsengaged position, and is also introduced into the check valve 412. Asthe selector lever is moved to the R position, the first solenoid valve400A is brought to its deenergized position, as indicated in thetable 1. Accordingly, the hydraulic pressure introduced into the checkvalve 412 is introduced into the low/reverse brake 44 through thehydraulic line 414, the hydraulic line 410, the space between the lands616 and 622 of the first fail-safe valve 600, and the hydraulic line638, thereby bringing the brake 44 to its engaged position. Thus,shifting into the reverse gear ratio is achieved.

As described above, the arrangement of the hydraulic control systemillustrated in FIG. 2 is such that the hydraulic pressure in thehydraulic line 88 is switched to the lowest first line pressure at theneutral, to the relatively high second line pressure at achievement ofshifting into the 1st and 2nd gear ratios, to the relatively low thirdline pressure higher than the first line pressure, but lower than thesecond line pressure at achievement of shifting into the 3rd and 4thgear ratios, and to the highest fourth line pressure at achievement ofshifting into the reverse gear ratio. The arrangement minimizes powerlosses due to driving of the pump 86 during the neutral in which theengine torque is not transmitted to the output shaft of thetransmission. In addition, by the arrangement, the line pressure isrelatively raised during shifting to the 1st and 2nd gear ratios inwhich relatively high torque is transmitted, to relatively increase theengaging forces of the respective engaging means, thereby preventingslippage. Further, since the transmission torque is relatively lowduring shifting to the 3rd and 4th gear ratios, the arrangement sets theline pressure to the relatively low level to reduce losses of the powerfor driving the pump 86. Moreover, by the arrangement, the line pressureis maximized during shifting to the reverse gear ratio in which thetransmission torque is the maximum, to prevent slippage of the engagingmeans.

The first through fourth solenoid valves 400A, 400B, 400C and 400D areduty-controlled to control the hydraulic pressures supplied to anddischarged from the respective engaging means corresponding respectivelyto the solenoid valves, thereby enabling smooth shifting of gear ratiosto be achieved.

The operation of the first and second fail-safe valves 600 and 700, whenmalfunction occurs in the electronic control system or in at least oneof the solenoid valves so that any one or more solenoid valve or valvesoperates or operate in error, will next be described.

The state will first be considered, where the first through thirdsolenoid valves 40A, 400B and 400C are not energized and the fourthsolenoid valve 400D is energized so that shifting into the 1st gearratio should be achieved. In such state, if the second solenoid valve400B malfunctions and is energized, the hydraulic line 316 is broughtinto communication with the hydraulic line 416. This causes thehydraulic pressure introduced into the hydraulic line 416 to beintroduced into the 2-4 brake 54 through the space between the lands 712and 718 of the second fail-safe valve 700, to bring the brake 54 to itsengaged position. At the same time, however, the hydraulic pressure inthe hydraulic line 416 is also introduced into the space between thelands 628 and 634 of the first fail-safe valve 600 through the hydrauliclines 640 and 644. Accordingly, the biasing force to the right due tothe hydraulic pressures acting upon the respective pressure receivingfaces 614 and 618 and, in addition thereto, the biasing force to theright due to the hydraulic pressures acting upon the respective pressurereceiving faces 626 and 630 are applied to the spool 636, and overcomethe biasing force to the left due to the hydraulic pressure acting uponthe pressure receiving face 606, so that the spool 636 is moved to theposition on the right-hand side in FIG. 2. Thus, the hydraulic line 638is brought into communication with the exhaust line 320. This releasesthe low/reverse brake 44, and brings only the 2-4 brake 54 and theunderdrive clutch 30 to their respective engaged positions, so thatshifting into the 2nd gear ratio is achieved.

If the fourth solenoid valve 400D malfunctions and is deenergized in thestate in which shifting into the 1st gear ratio should be achieved, thehydraulic line 316 is brought into communication with the hydraulic line424, so that the hydraulic pressure is introduced into the overdriveclutch 28 to bring the same to its engaged position. At the same time,the hydraulic pressure is introduced into the respective left-hand endhydraulic pressure chambers of the first and second fail-safe valves 600and 700 through the hydraulic line 512b. Accordingly, the spool 636 ofthe first fail-safe valve 600 is moved to the position on the right-handside in FIG. 2 under the biasing force due to the hydraulic pressuresacting respectively upon the pressure receiving face 632 and thepressure receiving faces 614 and 618, in a manner like that describedabove. At this time, the spool 732 of the second fail-safe valve 700 isnot switched to its discharge position on the right-hand side in FIG. 2.Since, however, the second solenoid valve 400B is deenergized and thehydraulic pressure is not supplied to the hydraulic line 416, the 2-4brake 54 is not brought to its engaged position. Accordingly, thehydraulic line 638 leading to the low/reverse brake 44 is brought intocommunication with the exhaust line 320 and, therefore, the low/reversebrake 44 is released and only the underdrive clutch 30 and the overdriveclutch 28 are brought to their respective engaged positions. Thus,shifting into the 3rd gear ratio is achieved.

If the second solenoid valve 400B is energized and the fourth solenoidvalve 400D is not energized in the state in which shifting into the 1stgear ratio should be achieved, the second solenoid valve 400B falls intoa malfunction state in addition to malfunction of only the fourthsolenoid valve 400D, so that the hydraulic line 316 is brought intocommunication with the hydraulic line 416. Accordingly, the hydraulicpressure is supplied to all of the hydraulic lines 410, 416, 422 and 424communicating the solenoid valves respectively with the engaging means28, 30, 44 and 54. Whereupon, the hydraulic pressure is applied to allof the pressure receiving face 728, the pressure receiving faces 722 and726 and the pressure receiving faces 710 and 714 of the second fail-safevalve 700, to bias the spool 732 to the right. Thus, the spool 732 isswitched to the position on the right-hand side in FIG. 2, to dischargethe hydraulic pressure in the hydraulic line 640 through the exhaustline 320, so that the 2-4 brake 54 is maintained released. In addition,since the hydraulic pressures are applied respectively to the pressurereceiving face 632 and the pressure receiving faces 614 and 618 of thefirst fail-safe valve 600, the biasing force to the right in FIG. 2 dueto the hydraulic pressures acting respectively upon these pressurereceiving faces becomes higher than that to the left in FIG. 2 due tothe hydraulic pressure acting upon the pressure receiving face 606.Thus, the spool 636 is switched to the position on the right-hand sidein FIG. 2, to bring the hydraulic line 638 into communication with theexhaust line 320, thereby maintaining the low/reverse brake 44 released.Accordingly, only the underdrive clutch 30 and the overdrive clutch 28are brought to their respective engaged positions, so that shifting intothe 3rd gear ratio is achieved.

The state will next be considered, where the first, second and fourthsolenoid valves 400A, 400B and 400D are energized and the third solenoidvalve 400C is not energized so that shifting into the 2nd gear ratioshould be achieved. In such state, if the first solenoid valve 400Abecomes deenergized, the hydraulic pressure is introduced to thepressure receiving faces 626 and 630 of the first fail-safe valve 600through the hydraulic line 644 and, in addition thereto, the hydraulicline 316 is brought into communication with the hydraulic line 410. Thiscauses the hydraulic pressure in the hydraulic line 410 to be introducedinto the space between the lands 616 and 622 of the first fail-safevalve 600. Thus, the biasing force to the right in FIG. 2 due to thesehydraulic pressures becomes higher than that to the left in FIG. 2 dueto the hydraulic pressure acting upon the pressure receiving face 606,to move the spool 636 to the position on the right-hand side in FIG. 2.Accordingly, communication between the hydraulic lines 410 and 638 isintercepted, and the hydraulic line 638 is brought into communicationwith the exhaust line 320, so that only 2-4 brake 54 and the underdriveclutch 30 are maintained engaged. Thus, the 2nd gear ratio ismaintained.

If the fourth solenoid valve 400D malfunctions and is deenergized in thestate in which shifting into the 2nd gear ratio should be achieved, thehydraulic line 316 is brought into communication with the hydraulic line424, so that the hydraulic pressure in the hydraulic line 424 isintroduced into the overdrive clutch 28 to bring the same to its engagedposition. At the same time, the hydraulic pressure is introduced intothe respective left-hand end hydraulic pressure chambers of the firstand second fail-safe valves 600 and 700 through the hydraulic line 512b.Whereupon, the hydraulic pressures are applied respectively to thepressure receiving face 728, the pressure receiving faces 722 and 726and the pressure receiving faces 710 and 714 of the second fail-safevalve 700. Accordingly, the biasing force due to these hydraulicpressures causes the spool 732 to be moved to the position on theright-hand side in FIG. 2, so that the hydraulic line 640 is broughtinto communication with the exhaust line 320, thereby releasing the 2-4brake 54. On the other hand, the hydraulic pressure is also introducedto the pressure receiving face 632 of the first fail-safe valve 600through the hydraulic line 512b. However, by the aforesaid switching ofthe second fail-safe valve 700, the hydraulic pressure is not introducedinto the space between the pressure receiving faces 626 and 630. Inaddition, the first solenoid valve 400A is energized so that thehydraulic pressure is not also introduced into the space between thepressure receiving faces 614 and 618. Accordingly, the spool 636 ismaintained at the position on the left-hand side in FIG. 2, but thehydraulic pressure is not supplied to the low/reverse brake 44 so thatthe brake 44 is maintained released. Thus, the underdrive clutch 30 andthe overdrive clutch 28 are brought to their respective engagedpositions and, therefore, shifting into the 3rd gear ratio is achieved.

If both the first and fourth solenoid valves 400A and 400D are notenergized in the state in which shifting into the 2nd gear ratio shouldbe achieved, the spool 636 of the first fail-safe valve 600 and thespool 732 of the second fail-safe valve 700 are both moved to theirrespective positions on the right-hand side in FIG. 2. This brings boththe hydraulic line 638 leading to the low/reverse brake 44 and thehydraulic line 640 leading to the 2-4 brake 54 into communication withthe exhaust line 320, so that the brakes 44 and 54 are brought to theirrespective released positions and the underdrive clutch 30 and theoverdrive clutch 28 are brought to their respective engaged positions.Thus, shifting into the 3rd gear ratio is achieved.

The state will next be considered, where the first solenoid valve 400Ais energized and the second through fourth solenoid valves 400B, 400Cand 400D are deenergized so that shifting into the 3rd ratio should beachieved. In such state, if the first solenoid valve 400A becomesdeenergized, the hydraulic line 316 is brought into communication withthe hydraulic line 410, so that the hydraulic pressure in the hydraulicline 410 is introduced into the space between the lands 616 and 622 ofthe first fail-safe valve 600. At this time, since the hydraulicpressure is also introduced to the pressure receiving face 632 throughthe hydraulic line 512b, the spool 636 is moved to the position on theright-hand side in FIG. 2, because of the relationship in pressurereceiving area between the pressure receiving faces 632 and 606. Thisbrings the hydraulic line 638 into communication with the exhaust line320, so that the hydraulic pressure is not supplied to the brake 44.Accordingly, the low/reverse brake 44 and the 2-4 brake 54 aremaintained released, and the underdrive clutch 30 and the overdriveclutch 28 are maintained engaged. Thus, the 3rd gear ratio ismaintained.

If the second solenoid valve 400B is energized in the state in whichshifting into the 3rd gear ratio should be achieved, the hydraulic line316 is brought into communication with the hydraulic line 416, so thatthe hydraulic pressure is introduced into the space between the lands712 and 718 of the second fail-safe valve 700. At this time, since thehydraulic pressures are also applied respectively to the pressurereceiving face 728 and the pressure receiving faces 722 and 726, thespool 732 is moved to the position on the right-hand side in FIG. 2 in amanner like that described above. This brings the hydraulic line 640leading to the 2-4 brake 54 into communication with the exhaust line320, so that the hydraulic pressure is not supplied to the brake 54.Thus, the 3rd ratio is maintained in a manner like that described above.

If the first solenoid valve 400A becomes deenergized and the secondsolenoid valve 400B becomes energized in the state in which shiftinginto the 3rd gear ratio should be achieved, the spool 636 of the firstfail-safe valve 600 and the spool 732 of the second fail-safe valve 700are both moved to their respective positions on the right-hand side inFIG. 2. The hydraulic lines 638 and 640 are brought into communicationwith the exhaust line 320, so that the hydraulic pressure is notsupplied to the brakes 44 and 54. Thus, the 3rd gear ratio ismaintained.

The state will next be considered, where the first through thirdsolenoid valves 400A, 400B and 400C are energized and the fourthsolenoid valve 400D is deenergized so that shifting into the 4th gearratio should be achieved. In such state, if the first solenoid valve400A becomes deenergized, the hydraulic line 316 is brought intocommunication with the hydraulic line 410. In the 4th gear ratio,however, the spool 636 of the first fail-safe valve 600 is beforehandmoved to the position on the right-hand side in FIG. 2, because of thehydraulic pressures acting respectively upon the pressure receiving face632 and the pressure receiving faces 626 and 630. Accordingly, thehydraulic pressure in the hydraulic line 410 is not supplied to thelow/reverse brake 44, so that only the 2-4 brake 54 and the overdriveclutch 28 are maintained engaged. Thus, the 4th gear ratio ismaintained.

If the third solenoid valve 400C becomes deenergized in the state inwhich shifting into the 4th gear ratio should be achieved, the hydraulicline 316 is brought into communication with the hydraulic line 422, sothat the hydraulic pressure is introduced into the underdrive clutch 30to bring the same to its engaged position. In addition, the hydraulicpressure is introduced into the space between the lands 724 and 730 ofthe second fail-safe valve 700 through the hydraulic line 736.Accordingly, the hydraulic pressures are applied respectively to all ofthe pressure receiving faces 722 and 726, the pressure receiving face728 and the pressure receiving faces 710 and 714 of the second fail-safevalve 700. This causes the spool 732 of the second fail-safe valve 700to be switched to the position on the right-hand side in FIG. 2, tobring the hydraulic line 640 into communication with the exhaust line320, so that the 2-4 brake 54 is released and only the underdrive clutch30 and the overdrive clutch 28 are brought to their respective engagedpositions. Thus, shifting into the 3rd gear ratio is achieved.

If the first and third solenoid valves 400A and 400C become deenergizedin the state in which shifting into the 4th gear ratio should beachieved, both the hydraulic lines 410 and 422 are brought intocommunication with the hydraulic line 316. However, the spool 636 of thefirst fail-safe valve 600 has already been switched to the position onthe right-hand side in FIG. 2, and the spool 732 of the second fail-safevalve 700 is switched to the position on the right-hand side in FIG. 2at the same time the hydraulic pressure is supplied to the hydraulicline 422 leading to the underdrive clutch 30. Accordingly, the 2-4 brake54 is released, and only the underdrive clutch 30 and the overdriveclutch 28 are brought into their respective engaged positions. Thus,shifting into the 3rd gear ratio is achieved.

The illustrated embodiment comprises the first and second fail-safevalves 600 and 700 and is arranged such that when three or more engagingmeans tend to be simultaneously brought to their respective engagedpositions, other engaging means are released leaving two engaging meansin their respective engaged positions. Thus, the illustrated embodimentis effective in that such a situation can be prevented in which three ormore engaging means are simultaneously brought to their respectiveengaged positions so that the transmission mechanism is locked tosuddenly lock the wheels of the vehicle from rotation or to cause thevehicle to become entirely unable to run.

In addition, the arrangement of the illustrated embodiment is such thatthe solenoid valves are provided correspondingly to the respectiveengaging means. With such arrangement, duty control of the respectivesolenoid valves enables the hydraulic pressures supplied to anddischarged from the respective engaging means to be controlled with ahigh accuracy, making it possible to reduce shifting shocks.

Further, the arrangement of the illustrated embodiment is such that thethree-way valves of ball valve type are employed as the first throughfourth solenoid valves as shown in FIG. 2, and the total number of thespool valves in the hydraulic control system is less than that of theconventional automatic transmission. Thus, it is possible to minimizethe probability that the solenoid valves and the spool valves becomestuck.

Moreover, the arrangement of the illustrated embodiment is such that thehydraulic pressure in the hydraulic line 88 is regulated to the firstline pressure by the pressure regulating valve 100 when the manuallyoperated valve 300 is in the N/P position. Such arrangement is effectivein that even if the rotational speed of the engine rises at the neutral,the hydraulic pressure in the hydraulic line 88 does not rise more thannecessary, making it possible to prevent noises from being generated.

Furthermore, the arrangement of the illustrated embodiment is such thatthe line pressure switching valve 500 is provided by which the hydraulicpressure is regulated to the relatively high second line pressure whenshifting into the 1st and 2nd gear ratios are achieved, and thehydraulic pressure is regulated to the relatively low third linepressure when shifting into the 3rd and 4th gear ratios are achieved.With such arrangement, the engaging forces of the respective engagingmeans can be brought to their respective levels corresponding to themagnitude of the transmission torque, making it possible to restrainlosses of the engine output for driving the pump 86 to the minimum.

The embodiment has been described as having the first fail-safe valve600 provided in the hydraulic line communicating the first solenoidvalve 400A and the low/reverse brake 44 with each other, and the secondfail-safe valve 700 provided in the hydraulic line communicating thesecond solenoid valve 400B and the 2-4 brake 54 with each other.However, the invention should not be limited to this specific form. Thatis, if three or more engaging means can be prevented from beingsimultaneously brought to their respective engaged positions, the firstfail-safe valve 600 may be provided in any one of the hydraulic linecommunicating the first solenoid valve 400A and the low/reverse brake 44with each other, the hydraulic line communicating the second solenoidvalve 400B and the 2-4 brake 54 with each other, and the hydraulic linecommunicating the fourth solenoid valve 400D and the overdrive clutch 28with each other. Further, the second fail-safe valve 700 may be providedin any one of the hydraulic line communicating the second solenoid valve400B and the 2-4 brake 54 with each other, the hydraulic linecommunicating the third solenoid valve 400C and the underdrive clutch 30with each other, and the hydraulic line communicating the fourthsolenoid valve 400D and the overdrive clutch 28 with each other. Thesecond fail-safe valve 700 should not be provided in the hydraulic linein which the first fail-safe valve 600 is provided. In this case, therecan also be obtained advantages similar to those of the illustratedembodiment.

Further, in the illustrated embodiment, since the first and secondfail-safe valves serving respectively as first and second switchingvalves are spool valves, the pressure receiving faces of the spoolvalves are utilized as detecting means for detecting the hydraulicpressures in the respective hydraulic lines communicating the solenoidvalves respectively with the engaging means, and as switching means forswitching the spool valves. However, the invention should not be limitedto the specific form. That is, pressure sensors serving as detectingmeans may be provided respectively in the above-mentioned hydrauliclines, and fail-safe valves formed, for example, by electromagneticswitching valves or the like may be switched between their respective ONand OFF positions in response to the outputs from the respectivesensors. Also in this case, advantages similar to those of theillustrated embodiment can be obtained.

Moreover, the arrangement of the first and second fail-safe valves 600and 700 in the illustrated embodiment is such that the hydraulicpressure in the hydraulic passage means 604 is applied to the pressurereceiving face 606 of the spool 636 and to the pressure receiving face708 of the spool 732, to normally bias the spools 636 and 732 in theleft-hand direction as viewed in FIG. 2. However, the invention shouldnot be limited to this specific form. Spring force may be substitutedfor the hydraulic force, to normally bias the spools 636 and 732 in theleft-hand direction as viewed in FIG. 2.

What is claimed is:
 1. A hydraulic control system for a vehicleautomatic transmission which comprises first and second engaging meansrespectively engagable for establishing different gear ratios, hydraulicpressure supplying means for supplying hydraulic pressure to causeengagement of each of said first and second engaging means, firsthydraulic passage means for connecting said hydraulic pressure supplyingmeans to said first engaging means, first shift means for switchingsupply and discharge of the hydraulic pressure to and from said firstengaging means in accordance with running conditions of the vehicle,second hydraulic passage means for connecting said hydraulic pressuresupplying means to said second engaging means, and second shift meansfor switching supply and discharge of the hydraulic pressure to and fromsaid second engaging means in accordance with the running conditions ofthe vehicle, said hydraulic control system comprising:first pressuredetecting means for detecting hydraulic pressure in said first hydraulicpassage means between said first shift means and said first engagingmeans; second pressure detecting means for detecting hydraulic pressurein said second hydraulic passage means between said second shift meansand said second engaging means; valve means coupled to said firsthydraulic passage means between said first pressure detecting means andsaid first engaging means and being movable between a supply positionwhere said valve means opens said first hydraulic passage means forsupplying the hydraulic pressure to be supplied to said first engagingmeans and a discharge position where said valve means discharges thehydraulic pressure supplied to said first engaging means; and switchingmeans coupled to said valve means for switching said valve means to movesaid valve means to said discharge position when said first pressuredetecting means detects that the hydraulic pressure in said firsthydraulic passage means is higher than a first predetermined value andsaid second pressure detecting means detects that the hydraulic pressurein said second hydraulic passage means is higher than a secondpredetermined value.
 2. The hydraulic control system of claim 1, whereinsaid valve means comprises a spool valve which includes a spool movablebetween supply and discharge positions which correspond respectivelywith said supply and discharge positions of said valve means andnormally biased toward said supply position under a predetermined force,and a plurality of lands integrally formed on said spool, said landshaving their respective pressure receiving sections including a firstpressure receiving section having applied thereto the hydraulic pressuresupplied from said first shift means to said first engaging means, forbiasing said spool toward said discharge position thereof, and a secondpressure receiving section having applied thereto the hydraulic pressuresupplied from said second shift means to said second engaging means, forbiasing said spool toward said discharge position thereof, said spoolbeing switched to said discharge position thereof when the hydraulicpressure acting upon said first pressure receiving section exceeds saidfirst predetermined value and the hydraulic pressure acting upon saidsecond pressure receiving section exceeds said second predeterminedvalue.
 3. The hydraulic control system of claim 2, wherein said pressurereceiving sections of the respective lands further includes a thirdpressure receiving section having normally applied thereto hydraulicpressure of a predetermined magnitude, for biasing said spool towardsaid supply position thereof.
 4. The hydraulic control system of claim3, wherein said predetermined magnitude of the hydraulic pressurenormally acting upon said third pressure receiving section, and saidfirst and second predetermined values are substantially equal to eachother, said third pressure receiving section having an effectivepressure receiving area larger than that of each of said first andsecond pressure receiving sections, but smaller than a sum of thepressure receiving areas of said respective first and second pressurereceiving sections.
 5. A hydraulic control system for a vehicleautomatic transmission which comprises first, second and third engagingmeans, said engaging means being combined to form a plurality of sets ofpairs, the engaging means in each set being simultaneously engaged toenable shifting into a plurality of gear ratios corresponding in numberto said sets to be achieved, hydraulic pressure supplying means forsupplying hydraulic pressure to cause engagement of each of said first,second and third engaging means respectively, first hydraulic passagemeans for connecting said hydraulic pressure supplying means to saidfirst engaging means, first shift means coupled to said first hydraulicpassage means for switching supply and discharge of the hydraulicpressure to and from said first engaging means in accordance withrunning conditions of the vehicle, second hydraulic passage means forconnecting said hydraulic pressure supplying means to said secondengaging means, second shift means coupled to said second hydraulicpassage means for switching supply and discharge of the hydraulicpressure to and from said second engaging means in accordance with therunning conditions of the vehicle, third hydraulic passage meansconnecting said hydraulic pressure supplying means to said thirdengaging means, and third shift means coupled to said third hydraulicpassage means for switching supply and discharge of the hydraulicpressure to and from said third engaging means in accordance with therunning conditions of the vehicle, said hydraulic control systemcomprising:first pressure detecting means for detecting hydraulicpressure in said first hydraulic passage means between said first shiftmeans and said first engaging means; second pressure detecting means fordetecting hydraulic pressure in said second hydraulic passage meansbetween said second shift means and said second engaging means; thirdpressure detecting means for detecting hydraulic pressure in said thirdhydraulic passage means between said third shift means and said thirdengaging means; valve means coupled to said first hydraulic passagemeans between said first pressure detecting means and said firstengaging means and being movable between a supply position where saidvalve means opens said first hydraulic passage means for supplying thehydraulic pressure to be supplied to said first engaging means and adischarge position where said valve means discharges the hydraulicpressure supplied to said first engaging means; and switching meanscoupled to said valve means for switching said valve means to move saidvalve means to said discharge position when said first pressuredetecting means detects that the hydraulic pressure in said firsthydraulic passage means is higher than a first predetermined value, saidsecond pressure detecting means detects that the hydraulic pressure insaid second hydraulic passage means is higher than a secondpredetermined value, and said third pressure detecting means detectsthat the hydraulic pressure in said third hydraulic passage means ishigher than a third predetermined value.
 6. The hydraulic control systemof claim 5, wherein said valve means comprises a spool valve whichincludes a spool movable between supply and discharge positions whichcorrespond respectively with said supply and discharge positions of saidvalve means and normally biased toward said supply position thereofunder a predetermined force, and a plurality of lands integrally formedon said spool, said lands having their respective pressure receivingsections including a first pressure receiving section having appliedthereto the hydraulic pressure supplied from said first shift means tosaid first engaging means, for biasing said spool toward said dischargeposition thereof, a second pressure receiving section having appliedthereto the hydraulic pressure supplied from said second shift means tosaid second engaging means, for biasing said spool toward said dischargeposition thereof, and a third pressure receiving section having appliedthereto the hydraulic pressure supplied from said third shift means tosaid third engaging means, for biasing said spool toward said dischargeposition thereof, said spool being moved to said discharge positionthereof when the hydraulic pressure acting upon said first pressurereceiving section exceeds said first predetermined value, the hydraulicpressure acting upon said second pressure receiving section exceeds saidsecond predetermined value, and the hydraulic pressure acting upon saidthird pressure receiving section exceeds said third predetermined value.7. The hydraulic control system of claim 6, wherein said pressurereceiving sections of the respective lands further includes a fourthpressure receiving section having normally applied thereto hydraulicpressure of a predetermined magnitude, for biasing said spool towardsaid supply position thereof.
 8. The hydraulic control system of claim7, wherein said predetermined magnitude of the hydraulic pressurenormally acting upon said fourth pressure receiving section, and saidfirst, second and third predetermined values are substantially equal toeach other, said fourth pressure receiving section having an effectivepressure receiving area larger than a sum of respective effectivepressure receiving areas of any two of said first, second and thirdpressure receiving sections, but smaller than a total sum of thepressure receiving areas of said respective first, second and thirdpressure receiving sections.
 9. A hydraulic control system for a vehicleautomatic transmission which comprises first, second and third engagingmeans respectively engagable for achieving three different forward gearratios, fourth engaging means engagable when achieving each of saidthree forward gear ratios, hydraulic pressure supplying means forsupplying hydraulic pressure to cause engagement of each of said first,second, third and fourth engaging means respectively, first hydraulicpassage means for connecting said hydraulic pressure supplying means tosaid first engaging means, first shift means coupled to said firsthydraulic passage means for switching supply and discharge of thehydraulic pressure to and from said first engaging means in accordancewith running conditions of the vehicle, second hydraulic passage meansfor connecting said hydraulic pressure supplying means to said secondengaging means, second shift means coupled to said second hydraulicpassage means for switching supply and discharge of the hydraulicpressure to and from said second engaging means in accordance with therunning conditions of the vehicle, third hydraulic passage means forconnecting said hydraulic pressure supplying means to said thirdengaging means, third shift means coupled to said third hydraulicpassage means for switching supply and discharge of the hydraulicpressure to and from said third engaging means in accordance with therunning conditions of the vehicle, and fourth hydraulic passage meansfor connecting said hydraulic pressure supplying, means to said fourthengaging means, said hydraulic control system comprising:first pressuredetecting means for detecting hydraulic pressure in said first hydraulicpassage means between said first shift means and said first engagingmeans; second pressure detecting means for detecting hydraulic pressurein said second hydraulic passage means between said second shift meansand said second engaging means; third pressure detecting means fordetecting hydraulic pressure in said third hydraulic passage meansbetween said third shift means and said third engaging means; fourthpressure detecting means for detecting hydraulic pressure in said fourthhydraulic passage means; first valve means provided in a portion of anyone of said first, second and third hydraulic passage means, saidportion thereof extending between the pressure detecting means coupledto the hydraulic passage means and the engaging means connected to thehydraulic passage means, said first valve means being movable between asupply position where said first valve means opens the hydraulic passagemeans for supplying the hydraulic pressure to be supplied to theengaging means and a discharge position where said first valve meansdischarges the hydraulic pressure supplied to the engaging means; secondvalve means provided in a portion of any one of said second, third andfourth hydraulic passage means in which hydraulic passage means saidfirst valve means is not provided, said portion thereof extendingbetween the pressure detecting means coupled to the hydraulic passagemeans and the engaging means connected to the hydraulic passage means,said second valve means being movable between a supply position wheresaid second valve means opens the hydraulic passage means for supplyingthe hydraulic pressure to be supplied to the engaging means and adischarge position where said second valve means discharges thehydraulic pressure supplied to the engaging means; first switching meanscoupled to said first valve means for switching said first valve meansto move said first valve means to said discharge position when thepressure detecting means coupled to the hydraulic passage means havingprovided therein said first valve means among said first, second andthird hydraulic passage means detects that the hydraulic pressure in thehydraulic passage means is higher than a predetermined value and thepressure detecting means coupled to at least one of the remaininghydraulic passage means detects that the hydraulic pressure in thehydraulic passage means is higher than a predetermined value; and secondswitching means coupled to said second valve means for switching saidsecond valve means to move said second valve means to said dischargeposition when the pressure detecting means coupled respectively to saidsecond, third and fourth hydraulic passage means detect simultaneouslythat the hydraulic pressures in the respective hydraulic passage meansare higher than their respective predetermined values.
 10. A hydrauliccontrol system for a vehicle automatic transmission which comprisesfirst, second, third and fourth engaging means for establishingdifferent forward gear ratios, hydraulic pressure supplying means forsupplying hydraulic pressure to cause engagement of each of said first,second, third and fourth engaging means respectively, first hydraulicpassage means for connecting said hydraulic pressure supplying means tosaid first engaging means, first shift means coupled to said firsthydraulic passage means for switching supply and discharge of thehydraulic pressure to and from said first engaging means in accordancewith running conditions of the vehicle, second hydraulic passage meansfor connecting said hydraulic pressure supplying means to said secondengaging means, second shift means coupled to said second hydraulicpassage means for switching supply and discharge of the hydraulicpressure to and from said second engaging means in accordance with therunning conditions of the vehicle, third hydraulic passage means forconnecting said hydraulic pressure supplying means to said thirdengaging means, third shift means coupled to said third hydraulicpassage means for switching supply and discharge of the hydraulicpressure to and from said third engaging means in accordance with therunning conditions of the vehicle, fourth hydraulic passage means forconnecting said hydraulic pressure supplying means to said fourthengaging means, and fourth shift means coupled to said fourth hydraulicpassage means for switching supply and discharge of the hydraulicpressure to and from said fourth engaging means, the arrangement beingsuch that a first gear ratio is achieved when said first and fourthengaging means are simultaneously engaged, a second gear ratio isachieved when said second and fourth engaging means are simultaneouslyengaged, a third gear ratio is achieved when said third and fourthengaging means are simultaneously engaged, a fourth gear ratio isachieved when said second and third engaging means are simultaneouslyengaged, said hydraulic control system comprising:first pressuredetecting means for detecting hydraulic pressure in said first hydraulicpassage means between said first shift means and said first engagingmeans; second pressure detecting means for detecting hydraulic pressurein said second hydraulic passage means between said second shift meansand said second engaging means; third pressure detecting means fordetecting hydraulic pressure in said third hydraulic passage meansbetween said third shift means and said third engaging means; fourthpressure detecting means for detecting hydraulic pressure in said fourthhydraulic passage means between said fourth shift means and said fourthengaging means; first valve means provided in a portion of any one ofsaid first, second and third hydraulic passage means, said portionthereof extending between the pressure detecting means coupled to thehydraulic passage means and the engaging means connected to thehydraulic passage means, said first valve means being movable between asupply position where said first valve means opens the hydraulic passagemeans for supplying the hydraulic pressure to be supplied to theengaging means and a discharge position where said first valve meansdischarges the hydraulic pressure supplied to the engaging means; secondvalve means provided in a portion of any one of said second, third andfourth hydraulic passage means in which hydraulic passage means saidfirst valve means is not provided, said portion thereof extendingbetween the pressure detecting means coupled to the hydraulic passagemeans and the engaging means connected to the hydraulic passage means,said second valve means being movable between a supply position wheresaid second valve means opens the hydraulic passage means for supplyingthe hydraulic pressure to be supplied to the engaging means and adischarge position where said second valve means discharges thehydraulic pressure supplied to the engaging means; first switching meanscoupled to said first valve means for switching said first valve meansto move said first valve means to said discharge position thereof whenthe pressure detecting means coupled to the hydraulic passage meanshaving provided therein said first valve means detects that thehydraulic pressure in the hydraulic passage means is higher than apredetermined value and the pressure detecting means coupled to at leastone of the remaining hydraulic passage means detects that the hydraulicpressure in the hydraulic passage means is higher than a predeterminedvalue; and second switching means coupled to said second valve means forswitching said second valve means to novel said second valve means tosaid discharge position when the pressure detecting means coupledrespectively to said second, third and fourth hydraulic passage meanssimultaneously detect that the hydraulic pressures in the respectivehydraulic passage means are higher than their respective predeterminedvalues.
 11. The hydraulic control system of claim 10, wherein said thirdgear ratio is a directly coupled gear ratio in which an input shaft andan output shaft of the transmission are substantially equal inrotational speed to each other, and said fourth gear ratio is anoverdrive gear ratio in which said output shaft is higher in rotationalspeed than said input shaft.